In the medical fields of cardiology and electrophysiology, many tools are used to assess the condition and function of a patient's heart, including the observed frequency, and morphology of the PQRST complex associated with a heart cycle. Such tools include classic external ECG systems for displaying and recording the characteristic lead ECG signals from skin electrodes placed on the patient's chest and limbs, ambulatory ECG Holter monitors for continuously recording the ECG or segments thereof from a more limited set of skin electrodes for a period of time, and more recently developed completely implantable cardiac monitors or cardiac pacemakers and implantable cardioverter/defibrillators (ICDs) having the capability of recording electrogram (EGM) segments or data derived from atrial and ventricular EGMS (A-EGMs and V-EGMs) for telemetry out to an external programmer for external storage and display.
Early automatic detection systems for automatic cardioverter/defibrillators relied upon the presence or absence of electrical and mechanical heart activity (such as intra-myocardial pressure, blood pressure, impedance, stroke volume or heart movement) and/or the rate of the electrocardiogram to detect hemodynamically compromising ventricular tachycardia or fibrillation.
Presently available pacemaker/cardioverter/defibrillator arrhythmia control devices employ programmable fibrillation interval ranges and tachycardia detection interval ranges, along with measurement of suddenness of onset and rate variability. For future generations of devices, numerous detection and classification systems have been proposed. Numerous patents, including U.S. Pat. No. 5,217,021 issued to Steinhaus et al., U.S. Pat. No. 5,086,772 issued to Larnard et al., U.S. Pat. No. 5,058,599 issued to Andersen and U.S. Pat. No. 5,312,441 issued to Mader et. Al., propose waveform morphology analysis systems for determining the type and origin of detected arrhythmias. Other patents, including U.S. Pat. No. 5,205,583 issued to Olson, U.S. Pat. No. 5,913,550 issued to Duffin, U.S. Pat. No. 5,193,535 issued to Bardy et al., U.S. Pat. No. 5,161,527 issued to Nappholz et al., U.S. Pat. No. 5,107,850 issued to Olive and U.S. Pat. No. 5,048,521, issued to Pless et al. propose systems for analysis of order and timing of atrial and ventricular events.
In the existing and proposed devices discussed above, one or two basic strategies are generally followed. A first strategy is to identify heart events, event intervals or event rates as they occur as indicative of the likelihood of the occurrence of specific types of arrhythmias, with each arrhythmia having a preset group of criteria that must be met as precedent to detection or classification. As cardiac events progress, criteria for identifying the various arrhythmias are all monitored simultaneously, with the first set of criteria to be met resulting in detection and diagnosis of the arrhythmia. A second strategy is to define a set of criteria for events, event intervals and event rates which is generally indicative of a group of arrhythmias, and following those criteria being met, analyzing preceding or subsequent events to determine which specific arrhythmia is present. An arrhythmia detection and classification system generally as disclosed in U.S. Pat. No. 5,342,402, issued to Olson et al., incorporated herein by reference in its entirety, uses both strategies together. In addition, numerous patents issued to Olson et al., including, for example, U.S. Pat. No. 5,545,186, U.S. Pat. No. 5,855,593, U.S. Pat. No. 5,991,656 U.S. Pat. No. 6,141,581, U.S. Pat. No. 6,178,350, U.S. Pat. No. 6,259,947, in addition to U.S. Pat. No. 6,052,620 issued to Gillberg et al., each incorporated herein by reference in their entireties, are directed to the use of a hierarchical rule based arrhythmia detection methodology based on a set of prioritized rules, each of the rules defining a plurality of criteria based upon characteristics of sensed depolarizations of heart tissue, each role being met when the criteria associated with the role are met.
In certain cases, patients utilizing implantable cardioverter/defibrillators tend to experience a number of spontaneous VT/VF episodes, or arrhythmia clusters, over a short period of time. For example, between approximately 75–90% of all VT/VF episodes occur in a form of clustering, typically having interdetection intervals less than one hour. Although the causes for the occurrence of such episodes in quick succession is unclear, myocardial ischemia, electrolyte imbalance, neurological disturbance, hormonal changes, and drugs are thought to be possible factors. While current implantable cardioverter/defibrillators treat specific, single VT/VF episodes, present implantable cardioverter/defibrillators do not attempt to detect arrhythmia clusters and to prevent the occurrence of future episodes that are associated with the detected arrhythmia cluster. Accordingly, what is needed is a method and device for automatically detecting the occurrence of arrhythmia clusters and preventing future episodes.